Instrumentation, air pollution abatement equipment, scientific instruments, medical devices such as needles and implants, fuel injection nozzles, spinnerettes and gas escapement orifices are some of the products that require extremely small holes, including those holes with other-than-round shapes. Diesel fuel injectors require holes between 0.005 and 0.010 inches in diameter.
The traditional method of small hole drilling involves a drill bit that is subject to easy breakage, and a sensitive drill press in the hands of a skilled operator. The cost is usually high with broken drills and scrapped parts. If the workpiece is a hardened metal, the problems are compounded.
Electric discharge machining (EDM) has been employed from time to time in the drilling of small diameter holes. EDM has worked to a degree even though the results are usually far from optimum since most EDM equipment is designed to handle work on a much larger scale. Amperages, spark frequencies and overcuts that are ideal for machining a die segment, leave much to be desired in the task of drilling a fine, accurate hole with no appreciable layer of recast and solidified material on the hole surface.
EDM, however, still offers numerous advantages in the drilling of small diameter holes. One such advantage is that the hardness of the workpiece to be drilled is irrelevant as long the material is electrically conductive and a spark can be forced to jump from an electrode to the workpiece. The rate of metal removal is a function of electrical conductivity and thermal characteristics of the workpiece. While extremely hard materials sometimes have a higher melting temperature, it is the melting temperature, not the hardness, that is the governing factor.
On the other hand, if the workpiece has no electric conductivity, the EDM process cannot be used. However, all metals and metallic alloys are electrically conductive to some degree and yield to the controlled cascade of sparks.
Another advantage of the EDM process is that when properly controlled, the EDM hole drilling process is very accurate and has a high degree of stability. Because there is no direct contact between the electrode and the workpiece, there are no mechanical forces of the type found in conventional drilling. In small hole work, there is frequently not even a flow of dielectric fluid into the gap area to set up mechanical forces. The energy utilized in the actual metal removal process is divided into a very high frequency sparks which are closely controlled with electronic systems now available.
With the EDM process there is no undue heat generation or any significant mechanical forces involved. Consequently, there is no part distortion. As a result, extremely thin and/or fragile parts can be successfully drilled with the EDM process.
Also, once the EDM job is set up, it can be completely cycled in an automatic fashion. As a result, skilled operators are not required.
The tool cost per hole is also extremely low. To drill holes under 0.015 inches in diameter, a tungsten alloy wire electrode which comes in spools is used. For sizes over 0.015 inches, straight rods are usually used. By way of example, typically hundreds of dollars of tungsten alloy wire will furnish enough electrode material to drill millions of holes in diesel fuel injector nozzles.
Still another advantage of the EDM process is the ability to vary the hole diameter within a limited range by simply changing current parameters without changing the electrode itself. By contrast, the diameter of mechanically drilled holes is determined by the diameter of the cutting tool. In order to resize the holes the size of the tool must be changed or a secondary operation must be performed. In EDM there is always an overcut comprising the spark gap between the electrode and the workpiece. The gap is a direct function of current flow and the frequency with which it is applied. In particular, the higher the current flow or the lower the frequency of sparking, the greater the gap. On normal EDM work, the gap may be anywhere from 0.0001 to 0.0003 inches on a side. Thus, in hole drilling work, the hole diameter is always larger than the tool itself. In small hole work the overcut is relatively small, but it can be closely controlled.
There are no burrs on the holes produced by the EDM process. Metal is eroded away in very minute globules to leave a non-directional type of surface finish. In the amperage and frequency ranges utilized in small hole EDM drilling, a recast surface is virtually non-existent.
The EDM process is also particularly susceptible to the use of the multiple lead principle to drill a number of holes simultaneously. While there are certain limitations that come about from the shape of the workpiece, the distance between the centers of holes and so on, the EDM process saves time.
In general, the EDM process affects many areas of cost saving, including direct tool cost and net machining time by a reduction of workpiece loss due to drill breakage.
Designers have discovered that better gas mixtures will be developed in certain instances where the orifices are made in other-than-round shapes. The common household gas range seems to give a better gas/air mixture if the gases are passed through a rectangular burner portion. Rectangular electrode materials can be utilized in EDM hole drilling units to generate "other-than-round" holes in workpieces. The same technique can be used to generate a "star" or other unusual hole shape if an electrode with the desired shape could be developed and utilized in a simple, yet cost-effective manner.
One method currently being used is to solder or otherwise fixedly attach an other-than-round electrode at one end of a round elongated shaft. However, this approach has many drawbacks, including the drawback of having to frequently change the electrode during the hole-forming process.